1. Field of the Invention
The invention relates generally to ink jet printing and in particular to measuring print cartridge characteristics to improve and maintain the quality of printed images.
2. Description of the Related Art
In contrast to laser printers, which use dry ink, static electricity, and heat to place and bond the ink onto the media; ink jet printers eject extremely small droplets of wet ink onto the media. Two common techniques used to eject these small droplets rely on either heat, in a thermal ink jet, or pressure waves, in a piezo electric ink jet, to dislodge each ink droplet. These small droplets are ejected from an array of nozzles, often smaller in diameter than a human hair. The nozzle is part of a jet which is the basic drop ejection element and includes the nozzle, the fluid feature under the nozzle, and the ejector, which is a resistor for thermal ink jet or a piezo element for piezo electric ink jet. Multiple jets are configured into a printhead, which also may contain control electronics.
Ink jet printers which rely upon heat to dislodge the ink are sometimes referred to as bubble jet printers. The term bubble jet comes from the formation of bubbles in the ink in response to the application of heat. Small resistors create this heat which causes the ink to locally vaporize and form a bubble. The resistors are formed utilizing thick or thin film technology on a substrate. Typically, one resistor per orifice or nozzle is used. Additionally, the printhead can have a thermal sensing resistor (TSR) and a bulk heater resistor for active printhead temperature control. As the bubble expands, some of the ink is pushed out of the nozzle onto the media.
To eject a drop from a jet of a printhead, the printer electronics supplies an electrical pulse to the resistor located in the jet on the printhead. The pulse energy is determined by pulse shape, pulse voltage, pulse width and resistance of the resistor. The level of drop ejection energy directly contributes to drop ejection quality. Good drop ejection quality is expected when drop ejection energy is higher than a critical energy. When drop ejection energy is slightly lower than the critical energy, unhealthy drops with small drop weight and low drop velocity are ejected. No drops are ejected when the energy is too low. Therefore the printer needs to supply high enough energy to achieve good drop ejection quality.
Printers that rely on pressure waves are known as piezoelectric ink jet printers. Piezoelectric ink jet technology uses piezo elements for drop ejection. Under application of electrical potential, the piezo element is deformed. The dimensional change of the piezo element between the energized and resting states is controlled to generate pressure waves, which cause drop ejection. Different implementations can be designed, such as “shared wall”, “shear mode”, “bender”, and “piston” types. Electrically, a piezo element has electrical capacitance as a physical parameter. The capacitance is a good indicator of the quality of the piezo element.
Another important parameter of a piezo element in an ink jet printhead is the resonance frequency. Since the piezo element is mechanically coupled with the jet, the resonance frequency, which is measured electrically, is an indicator of the state of the piezo element and the fluid chamber of the jet. For example, an empty chamber or a clogged chamber will have different resonance frequencies. Drop ejection pulse is key to drop ejection quality of piezoelectric ink jet. The drop ejection pulse includes pulse shape, voltage, and pulse width. Though no heat is generated from the drop ejection in a piezoelectric printhead, drop weight can vary due to the environmental temperature. Printhead temperature control can be implemented, similar to the thermal ink jet printhead, for controlled drop weight or dot size on media. Methods of evaluating piezo elements are described in U.S. patent application Ser. No. 09/184,466, entitled Faulty Ink Ejector Detection In an Ink Jet Printer, now, U.S. Pat. No. 6,375,299, which is hereby incorporated in its entirety by reference.
For ink jet technology, images are made up from droplets of ink of different primary colors on media. The quality of the droplets contributes greatly to the image quality. The ink and media compatibility is another important factor. As the image quality and throughput of ink jet printers improved, they have become competitive with more traditional graphic arts production processes. Such improvements have allowed ink jet printers to become widely used in the graphic arts industry. To satisfy such users and optimize image quality, manufacturers maintain strict quality controls for a newly fabricated ink jet printer. However, wear and replacement of disposable components over time, such as the printhead or cartridge, may degrade image quality. The rigorous demands of the graphic arts industry has led ink jet printer manufacturers to focus on improving the quality of the printed image throughout the printer's usable life.
It can be appreciated that many different parameters affect the print quality achievable in ink jet printing. While ambient environmental conditions along with the selected type of ink and media may affect the result of the print process, the performance of the printhead is critical to good image quality. If one or more of the jets of the printhead is not ejecting the correct amount of ink at the right time, image quality significantly suffers.
With respect to the printhead, a variety of monitoring techniques have been developed to detect malfunctioning ink jet nozzles and warn the operator or compensate for the malfunctioning jet in some way. In most of these monitoring techniques, only jets which are not expelling ink at all, or “open” jets, can be detected. In some cases, this is accomplished by optical monitors which detect droplets of ink as they are expelled. This detection technique is complicated, and typically cannot detect jets which may be expelling some ink, but not the correct amount. Thus, these monitoring techniques are unable to provide the printer with enough information to allow it to adequately compensate for a poorly performing jet.